Position control apparatuses are used to drive systems, such as machining tools and lithography stages, in accordance with a defined path. The path may have one direction, e.g., the X coordinate, or two directions, e.g., the X and Y coordinates. Typically in a lithography apparatus for semiconductor wafer processing, a stage is used to position a mask (reticle) in two dimensions, while a separate stage is used to position the plate (wafer). The stages are moved relative to a base structure which supports the stages and other components such as a source of radiant energy and a projection lens to focus the energy. The stages may be moved in a step-and-repeat pattern or in a scanning pattern, in which the stages maintain a constant velocity during exposure.
A position control apparatus used to drive and position stages are known; see U.S. Pat. Nos. 5,184,055 and 4,952,858. Generally, position control apparatuses are concerned with compensating forces to limit vibration of the stages. This is particularly true with a step-and-repeat system where there are many accelerations (and decelerations) of the stages being driven. To limit vibrations or resonance of the system, the position control apparatuses use acceleration as a feedback control mechanism.
FIG. 1 is a block diagram showing a position control apparatus 10 which limits vibrations in the controlled system 16. Position control apparatus 10 has a control circuit 12 which receives a positioning error signal from node 14. The positioning error signal represents the difference between the desired position of the controlled system and the actual current position. The actual position may be determined by an interferometer or other appropriate measurement device. The control circuit 12 receives the positioning error signal and produces a control signal. The controlled system 16, such as an X-Y stage, receives a signal from node 18, which represents the difference between the control signal from control circuit 12 and an acceleration feedback signal from feedback circuit 20. An acceleration sensor (not shown), which provides an acceleration feedback signal, is mounted on controlled system 16. Feedback circuit adjusts the gain of the acceleration feedback signal and provides the resulting signal to node 18. A position gauge 22, such as a laser interferometer provides a signal representing the current actual position of controlled system 16. The signal from position gauge 22 is then fed back to node 14.
The position control apparatus of FIG. 1 thus moves controlled system 16 to the new desired location. Use of an acceleration feedback loop permits modifying the control signal from control circuit 12 in response to the current acceleration of controlled system 16. Consequently, controlled system 16 is moved such that the resonance of controlled system 16 is reduced.
Where the stages are moved in a scanning pattern, however, the stages are moved at a constant velocity. The stages are subject to acceleration primarily at the beginning and end of the scan. Consequently, in a scanning system, problems created by the resonance of the system are not as prevalent as found in a step-and-repeat system.
However, in a scanning system the stages must be moved in a synchronous fashion. The position control apparatus must drive the stages at a constant velocity while maintaining the alignment of the stages. Where extreme precision is required, such as in a microlithographic system that produces images on the sub-micron scale, any misalignment of the stages will result in defects in the exposed image. Misalignment of the stages is known as synchronous error. Thus, a position control apparatus that drives stages at a constant velocity with a minimum of synchronous error is needed.
Further, because exposure of the plate can only occur while the system is at a constant velocity, the system must wait for the stages to accelerate to the constant velocity at the beginning of the scan before beginning the exposure and must end the exposure prior to decelerating the stages at the end of the scan. The time spent accelerating and decelerating the system is known as the settling time of the system and is a limitation on the throughput of the system. To increase the efficiency of the system, it is desirable to reduce the settling time.